The present invention relates generally to inductive plasma processors including an RF excitation coil and more particularly to such a processor including a shielded current sensor coupled to a low voltage portion of a branch including a winding of the coil.
One type of processor for treating workpieces with an RF plasma in a vacuum chamber includes a coil responsive to an RF source. The coil responds to the RF source to produce electromagnetic fields that excite ionizable gas in the chamber to produce a plasma. Usually the coil is on or adjacent to a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. The excited plasma interacts with the workpiece in the chamber to etch the workpiece or to deposit material on it. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral planar coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the RF source via an impedance matching network. Coils produce oscillating RF fields having magnetic and electric field components that penetrate through the dielectric window to accelerate electrons and ions in a portion of the plasma chamber close to the window. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by the current at each point of the coils. The inductive component of the electric field is produced by the time varying magnetic field, while the capacitive component of the electric field is produced by the RF voltage in the coils. The inductive electric field is azimuthal while the capacitive electric field is vertical downward to the workpiece. The current and voltage differ at different points because of transmission line effects of the coil at the frequency of the RF source.
For spiral designs as disclosed by and based on the Ogle ""458 patent, the RF currents in the spiral coil are distributed to produce a ring shaped electric field resulting in a toroidal plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the ring shaped region where plasma density is greatest tends to smear out plasma non-uniformity and increases plasma density in the chamber center just above the center of the workpiece. However, the diffusion alone generally can not sufficiently compensate plasma losses to the chamber walls and plasma density around the workpiece periphery can not be changed independently. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrals in the plasma further prevent substantial diffusion of the plasma charged particles from the toroidal region. As a result, there is a relatively high plasma density in a ring like region of the workpiece but low plasma densities in the center and peripheral workpiece portions.
These different operating conditions result in substantially large plasma flux (i.e., plasma density) variations between inside the toroid and outside the toroid, as well as at different azimuthal angles with respect to a center line of the chamber that is at right angles to the plane of the workpiece holder (i.e., chamber axis). These plasma flux variations result in a substantial standard deviation, i.e., in excess of six percent, of the plasma flux incident on the workpiece. The substantial standard deviation of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e., different portions of the workpiece are etched to different extents and/or have different amounts of materials deposited on them.
Our co-pending, commonly assigned application entitled xe2x80x9cINDUCTIVE PLASMA PROCESSOR HAVING COIL WITH PLURAL WINDINGS AND METHOD OF CONTROLLING PLASMA DENSITY,xe2x80x9d (Lowe Hauptman Gilman and Berner Docket No. 2328-050) discloses an arrangement for providing greater uniformity of plasma flux incident on the workpiece. In a preferred arrangement disclosed in the co-pending application, the coil current amplitude is measured to verify that the correct current is flowing in the coil and to assist in providing control, if necessary, for the plasma density.
In the past, electrical parameters to assist in controlling the operation of RF coil-excited plasma processors have involved measuring parameters, such as coil current amplitude, voltage amplitude and the phase angle between the voltage and the current. Other parameters have also been monitored, such as the forward and reflected powers. These parameters have been measured at high voltage portions of circuitry driving the matching network or coil. As a result, the current sensors, which typically include a toroidal coil surrounding a lead in a branch including a coil winding, are usually coupled to noise-inducing RF fields. As a result, the sensors are affected by the ambient RF fields and tend to derive inaccurate indications of the coil current.
Shielding of the coil toroidal winding in these prior art configurations to reduce or substantially eliminate the electric noise coupled to the sensor toroidal coil has usually not been feasible. This is because the shield, in order to be effective, must be grounded and in close proximity to the sensor toroidal coil which is coupled to high voltage portions of the circuitry. The high voltage and shield proximity requirements are likely to result in a discharge between the shield and coil or between the shield and other parts of the circuitry driving the coil. In addition, the grounded shield can be strongly coupled in proximity to the high voltage and can greatly perturb the electric field distribution.
It is, accordingly, an object of the present invention to provide a new and improved inductive plasma processor having an RF excitation coil with an improved sensor for the current flowing in the coil.
An additional object of the invention is to provide an inductive plasma processor including an RF plasma excitation coil with an improved high accuracy current sensor.
Another object of the invention is to provide an inductive plasma processor including an RF plasma excitation coil with a current sensor that is shielded from electromagnetic fields and which is arranged so that the shield is not particularly subject to inducing a breakdown, even though it is grounded.
According to the invention, an inductive processor includes an RF plasma excitation coil including a winding having an input terminal and an output terminal. An RF source coupled to the coil supplies the input terminal with RF excitation current. The winding has one end connected in series with the RF source and matching network and a second end connected by a lead to ground. A current sensor is coupled to the lead, and surrounded by a grounded shield to prevent ambient RF fields from being coupled to the sensor.
Preferably, the coil includes plural windings, each in a separate branch and including an input terminal and an output terminal. Each of the input terminals is connected to be driven in parallel by the RF source and the matching network. Each of the output terminals is connected by a separate lead to ground. A separate current sensor is coupled to each of the ground leads and a shield arrangement coupled with each sensor decouples RF fields from the sensors.
In a preferred configuration, each branch includes a capacitor connected between the coil output terminal of the branch and the ground lead. The current sensor is preferably placed between the capacitor and the ground lead, such that the RF voltage, as well as the RF fields, are substantially close to zero, resulting in minimum electromagnetic interference (EMI) to the current sensor.
Typically, the sensor includes a toroidal structure, e.g., a toroidal coil, a toroidal magnetic core, as well as rectifying and filtering circuitry, and the grounded shield substantially surrounds the toroidal structure.